A tension spring is generally composed of solid tension coil spring in its greater part, but such tension coil spring is always required to have high severity such as in facing angle between the loops formed in both ends, shape of such loop, number of turns between these loops and diameter of coil etc. to be provided to suit place and purpose of application, and it is now an immediate need to develop an efficient and precise apparatus for manufacturing solid tension coil springs satisfying such requirements.
In the field of coil spring manufacturing, following three kinds of method for manufacturing a solid coil tension spring have been available.
(1) A method of executing four steps of forming the first loop, forming the coil portion and controlling the loop angle, forming the second loop and cutting with a single unit of the coil shaper while the wire material is intermittently sent with pressure. PA1 (2) A method including the steps; First, loop is formed; then the wire material is sent under pressure and the coil portion is formed, loop angle is controlled and cutting is carried out; upon completion of cutting, such coil spring is moved to the next stage in order to form the second loop (in this case, the first loop of the next coil spring is formed simultaneously with the second loop). PA1 (3) A method including the steps; The wire material is sent under pressure and the coil portion is formed followed by control of coil end angle; when cutting completes, such coil spring is sent to the next stage and the first loop is formed in such a stage; then such coil spring is sent to the next stage and such coil spring is placed reversely and caught again; thereafter the coil is sent to the next stage in order to form the second loop (in this case the respective steps are carried out simultaneously in each stage and in the coil portion forming step). PA1 first, a solid coil portion is formed by the coil spring forming machine of such a type that the bending dies is used for solid coil formation, namely by the coil spring forming machine of such a type that the wire material is sent by the pressurized roller and abuts in contact with the bending dies, then sequentially bent into coil and when once formed into a coil the coil portion is automatically subjected to the spring back because nothing abuts in contact with the coil portion and the number of turns or shape of coil no longer change; PA1 in this coil forming process, if the control for the number of turns of solid coil spring is carried out in such a manner that the control contact type sensor is located at the predetermined position and the pressurized sending of wire material is immediately stopped when the end of wire material of the formed coil portion abuts in contact with such control contact type sensor, such solid coil spring is cut at the specified position of the formed coil since the formed solid coil accurately has the specified number of turns; and PA1 the solid coil spring which is cut in the specified number of turns is held at the center thereof and is transferred to the next stage and the first and second loops are simultaneously formed at both ends of such solid coil spring in this transfer process.
Investigations from various aspects for such existing methods of manufacturing solid coil spring have proved that the respective methods have the defects as explained below.